Democrats have another headache in their struggle to keep control of the Senate this year. Senator Angus King, a Maine independent who caucuses with Democrats, is now saying it’s possible he could switch sides and help the Republicans form a Senate majority.

“I’ll make my decision at the time based on what I think is best for Maine,” King told the Hill newspaper yesterday.

King usually votes with Democrats, but this week he joined the GOP in blocking a Democratic bill requiring companies to explain pay disparities between the sexes and making it easier for those claiming discrimination to file lawsuits.

Control of the Senate may come down to one seat this fall. If the GOP wins six seats they will elect a majority leader and chair all committees. But if they gain only five seats and the Senate is tied at 50 to 50, Vice President Joe Biden would break the tie in favor of the Democrats. In 2001, a tie also occurred and the two parties worked out a partial power-sharing agreement.

Senator King has said Maine’s status as a small state makes it important for him to be part of any Senate majority. “In the situation where one party has a clear majority and effectiveness is an important criteria, affiliating with the majority makes the most sense,” King said when he ran for his first term in 2012.

King began his career as a liberal Democrat, but has long demonstrated flashes of independence. He ran for governor in 1994 as an independent saying, “Sometimes the best thing the government can do is get out of the way.” He defeated candidates of both major parties, eventually serving two full terms before retiring in 2002. As governor, King trimmed state employees and the budget and cut the time required for environmental permits dramatically.

When he ran for the Senate, he went out of his way to appeal to both conservatives and liberals. The Almanac of American Politics reports that “his Senate campaign headquarters prominently featured two photographs side by side: one of former Republican President Ronald Reagan, and the other of former Democratic Attorney General Robert Kennedy.”